1. Field of the Disclosure
Aspects of the disclosure are related to electroencephalography (EEG), and more specifically, to a method and system for fully automated localization of electroencephalography electrodes.
2. Description of the Related Art
Commonly referred to as “EEG,” electroencephalography is used both clinically and in research applications. The measurements that it makes on the scalp arise from sources deep in the brain, but determination of those source locations is difficult, and is limited significantly by indeterminacy of the electrode locations both with respect to each other and to the brain.
For the most part, the methods in use for localizing scalp electrodes are crude. Simple landmarks, such as the ears, the midline, the nasion (a notch above our nose), are used in placing the electrodes, but after applying the electrodes themselves there is still considerable variability. Most importantly, the relationship of these phrenological features of the head is related only loosely to the location of the electrodes with respect to the brain.
There are a variety of methods that attempt to improve electrode localization. For example, Polhemus (http://www.polhemus.com) makes a digitizing wand with which the user can essentially point to each electrode and have its location digitized with respect to other features of the head, typically the same points used in the fully manual approach. Another commercial device, the Electrical Geodesics, Inc. (http://www.egi.com) Geodesic Photogrammetry System, uses an array of cameras whose location is well known. With this system a user can point to each electrode in the pictures on a computer screen thereby identifying its location. These methods are relatively precise, but extremely tedious. Their principal value is that the electrodes are better localized relative to scalp features, but they make no direct reference to the brain. Further, they do not account for the common problem that electrode locations shift when a patient changes position (e.g., lies down on a bed). Nevertheless, such systems command high prices in the market.
There have also been attempts to determine electrode location from magnetic resonance imaging (MRI). Those methods relied on image intensity and located exogenous markers attached to each electrode, such as vitamin A or E capsule rather than the electrodes themselves. These image-guided approaches to date were performed on relatively sparse (˜32 electrode) arrays compared to the dense (128+ electrode) arrays currently available. In high-density arrays image distortions due to variations in magnetic susceptibility, and signal losses, are inevitable from the high electrode count and their respective wiring.